Strip casting aluminum alloy

ABSTRACT

AN ALUMINUM ALLOY COMPRISING, BY WEIGHT, 2.4 TO 3.2% MAGNESIUM, 0.6 TO 0.8% IRON, 0.2 TO 0.3% SILICON, 0.2 TO 0.3% COPPER, 0.2 TO 0.3% MANGANESE, 0.005 TO 0.025% TITANIUM, AND 0.005 TO 0.025% BORON, AND LESS THAN 0.01% CHROMIUM, THE BALANCE BEING ALUMINUM, IS E ESPECCIALLY USEFUL SINCE IT CAN BE FORMED BY STRIP CASTING. THE ALLOY POSESSES A COMBINATION OF STRENGTH AND FORMABILITY WELL SUITED FOR ALUMINUM CAN ENDS AND DRAWN AND IRONED CAN BODIES.

United States Patent Office 3,834,900 Patented Sept. 10, 1974 3,834,900 STRIP CASTING ALUMINUM ALLOY Albert Joseph Klein, Arlington Heights, Ill., assignor to American Can Company, Greenwich, Conn. No Drawing. Filed Oct. 16, 1972, Ser. No. 297,828 Int. Cl. C22c 21/02 US. Cl. 75-142 6 Claims ABSTRACT OF THE DISCLOSURE An aluminum alloy comprising, by weight, 2.4 to 3.2% magnesium, 0.6 to 0.8% iron, 0.2 to 0.3% silicon, 0.2 to 0.3% copper, 0.2 to 0.3% manganese, 0.005 to 0.025% titanium, and 0.005 to 0.025% boron, and less than 0.01% chromium, the balance being aluminum, is especially useful since it can be formed by strip casting. The alloy possesses a combination of strength and formability well suited for aluminum can ends and drawn and ironed can bodies.

BACKGROUND OF THE INVENTION This invention relates to an aluminum alloy composition, and more particularly to an aluminum alloy containing 2.4 to 3.2% by weight magnesium suitable for strip casting and use in can making.

Recent years have seen an increasing use of aluminum in the manufacture of cans. The introduction of two piece cans has created a need for aluminum alloy sheet for forming the can body that not only possesses the required combination of strength and formability properties, but is also economical. Economical considerations would require that the molten alloy be cast into a thin web one inch or less thick (strip casting), so that subsequent rolling is reduced to a minimum. By strip casting into webs 1 inch or less thick, it is possible to eliminate the step of hot rolling. The continued use of easy open can ends, such as the integral drawn rivet pull tab feature disclosed in US. Pat. 3,191,797, similarly requires an economical, easily processed aluminum alloy.

An aluminum alloy contemplated for use as a can component must have sufficient strength to safely contain the material within the can, and this strength must be retained after exposure to the baking temperature employed in curing the coatings applied to the can components. The alloy also must be sufficiently workable to be fashioned into a can body or an easy open end, such as disclosed in the above-mentioned US. patent.

The alloy disclosed in US. Pat. 3,502,448 is representative of the types of alloys now being processed into can components. None of the presently available alloys having the requisite strength and formability can be provided by strip casting, wherein coils less than one inch thick may be cast. Continuous strip casting is a cheaper means for producing the alloy sheet needed for can components than conventional slab casting, since the initial thickness of the strip cast alloy is so much less than the slab cast alloy, thus reducing the time and effort required to roll the cast coil into sheet.

Until now, it has been believed that an aluminum alloy containing more than 3% total alloying could not be made by strip casting. This belief was based on the idea that more than 3% alloying caused too wide a temperature range of solidification. With a wide solidification range it is likely that shrinkage problems will develop when the thin web undergoes a relatively rapid solidification, thereby causing the cast web of alloy to crack.

It had also been the belief of those skilled in the art that in order for an aluminum alloy to possess suflicient strength, magnesium should be employed, and that at least 4-5 by weight magnesium was required. Applicant has discovered that 4% minimum magnesium content is required only for slab casting, which is a relatively thick casting (nominally about 1 foot) having a relatively slow solidification process, which permits the magnesium to become less uniformly dispersed. The non-uniform dispersion in slab casting creates the need for a 4% minimum magnesium level on a bulk scale in order that any given minute section of the slab cast web will contain an adequate amount of magnesium for sufiicient strength. Applicant has discovered that a strip cast aluminum alloy with a lesser quantity of magnesium can achieve substantially the same strength level as a slab cast alloy having a greater quantity of magnesium. The faster solidification in the strip cast web enables the magnesium to be more uniformly dispersed, thereby lessening the amount of magnesium that need be employed, which results in large material cost savings.

The novel alloy disclosed herein, as noted, is particularly useful for easy open can ends. Increasing the strength of an alloy generally increases the brittleness, and a strong, brittle alloy with suiiicient formability is ideal for easy open ends. The instant invention thus provides a strong alloy with suflicient ductility that can be formed into an easy open end that is as strong as present commercial ends, but yet tears open easily.

SUMMARY OF THE INVENTION The instant invention provides an aluminum alloy suitable for strip casting which comprises, by weight, 2.4 to 3.2% magnesium, 0.6 to 0.8% iron, 0.2 to 0.3% silicon, 0.2 to 0.3% copper, 0.2 to 0.3% manganese, 0.005 to 0.025% titanium, 0.005 to 0.025% boron, and less than 0.01% chromium, the balance being aluminum.

DESCRIPTION OF THE PREFERRED EMBODIMENT The compositional range set forth in the preceding paragraph represents an alloy which, if cold rolled to H19 temper (usually achieved by an -90% reduction in thickness), possesses a tensile strength of at least 55,000 p.s.i., a yield strength of at least 50,000 p.s.i. (an indication of sufficient strength for can making purposes) and an elongation in two inches of about 3.1 to 3.6% (a measure of suflicient formability for can making purposes). Table I, below, indicates a preferred range of composition and a target composition, in percent by weight, which could be deemed the preferred embodiment of the instant invention:

If the amounts of magnesium, iron, silicon, copper, or manganese fall below the amount called for in the broad range (see Summary of the Invention), the resultant alloy will be too weak for purposes contemplated by the instant invention. If the magnesium employed exceeds the specified levels it becomes difiicult to get solidification of the alloy after it is strip cast. If the iron, silicon, copper, or manganese exceed the desired levels, alloy constituents may form which tend to cause brittleness and/or fracturing. Chromium is an impurity that is to be minimized to avoid casting defects. Titanium and boron are grain refiners and if they are employed in insufiicient amounts the grains of the alloy tend to become too coarse, resulting in poor alloy engineering properties.

In processing the novel alloy composition of the instant invention into sheet material by strip casting, it is to be noted that no hot rolling is necessary. The aluminum and alloying elements are charged into a melting furnace and then transferred into a holding furnace from which a stream of the alloy is fed into a conventional strip caster which solidifies a web between 0.25 and 1.0 inch thick (usually about 0.25 inch). The web is then upcoiled at about 200 to 300 F. and allowd to cool. The web is then cold rolled to effect a reduction in thickness of about 30%, after which it is annealed in an atmosphere controlled furnace. The web is cold rolled again to effect a reduction in thickness of about 80 to 90% to finish gauge, thereby imparting an H19 temper (extra hard) to the web.

The strength of the novel alloy of the instant invention (sheet A) is best illustrated by comparing it with an alloy (sheet B) presently employed by the can making industry. Table II provides a chemical analysis in percent by weight:

TABLE II 037 Balance TABLE III Properties as rolled A B Tensile strength, p.s.i 57, 900 64, 300 Yield strength, p.s.i 51, 60 55, 400 Percent elongation in 2 inches 3. 2 6. 2

Although the novel alloy is not as strong as the presently commercial alloy, it does exceed 55,000 p.s.i. tensile strength, which is required for can making purposes. Simi- 'larly, the elongations are not as high for the novel alloy as for the presently commercial alloy but are considered adequate for manufacture of easy open ends.

Table III indicates the properties of the alloy sheet in the as rolled condition. However, these sheets must be exposed to coating :and baking processes, and it is after such exposure that the properties of the sheet are of greatest importance, since these are the properties which will be exhibited by the sheet when provided as a can end or body. The properties listed in Table III are indicated in Table IV after the sheet has been coated and Percent elongation in 2 inches 6. 8 10. 7

Although the strength and formability of the novel alloy A are not as high as the commercial alloy, they are quite sufficient for can manufacturing purposes.

In addition to standard tensile tests, another measure of the strength capacity of a sheet to serve as a can end is the pressure deflection characteristic. This characteristic is determined by providing a beverage can with its end fashioned from the sheet being tested. Pressure is applied within the can and gradually increased until the can end buckles and plastically deforms. The pressure at which the can end buckles is then designated as the buckle pressure. In buckling tests comparing sheets A and B, sheet A is capable of achieving the same strength at 0.0140 inch gauge as sheet B is at 0.0135 inch gauge. This additional thickness is a small price to pay for being able to eliminate the step of hot rolling from the mill processing and being able to strip cast the alloy.

It is understood that the foregoing general and detailed descriptions are explanatory of the present invention and are not to be interpreted as restrictive of the scope of the following claims.

What is claimed is:

1. An aluminum alloy, suitable for strip casting, consisting essentially of 2.4 to 3.2% by weight magnesium, 0.6 to 0.8% by weight iron, 0.2 to 0.3% by weight silicon, 0.2 to 0.3% by weight copper, 0.2 to 0.3% by weight manganese, 0.005 to 0.025% by weight titanium, 0.005 to 0.025% by weight boron, and less than 0.01% by weight chromium, the balance being aluminum.

2. The alloy of Claim 1 which, when cold rolled to an H19 temper possesses a tensile strength of at least 55,000 p.s.i., a yield strength of at least 50,000 p.s.i., and an elongation, in 2 inches, of about 3.1 to 3.6%.

3. The alloy of Claim 1 consisting essentially of 2.65 to 2.95% by weight magnesium, 0.63 to 0.68% by weight iron, 0.23 to 0.27% by Weight silicon, 0.23 to 0.27% by weight copper, 0.23 to 0.27% by weight manganese, 0.01 to 0.02% by weight titanium, 0.01 to 0.02% by weight boron and less than 0.001% by weight chromium, the balance being aluminum.

4. The alloy of Claim 3 which, when cold rolled to an Hl9 temper possesses a tensile strength of at least 55,000 p.s.i., a yield strength of at least 50,000 p.s.i., and an elongation, in 2 inches, of about 3.1 to 3.6%.

5. The alloy of Claim 3 consisting essentially of about 2.8% by Weight magnesium, about 0.65% by weight iron, about 0.25% by weight silicon, about 0.25% by weight copper, about 0.25% by weight manganese, about 0.15% by weight titanium, about 0.015% by weight boron, and 0.000% by weight chromium, the balance being aluminum.

6. The alloy of Claim 5 which, when cold rolled to an H19 temper possesses a tensile strength of at least 55,000, p.s.i., a yield strength of at least 50,000 p.s.i., and an elongation, in 2 inches, of about 3.1 to 3.6%.

References Cited UNITED STATES PATENTS RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 148-32 

